The invention was originally designed for use with long haul-type trucks, which have a tractor and semi-trailer combination. By "semi-trailer" we refer to the type of trailer commonly found on combination trucks. The semi-trailer mounts on a fifth wheel of a tractor. In the case of "doubles" and "triples" (two or three trailers), the fifth wheel is part of a dolly which is attached to the back end of a leading trailer. For this reason, the second and subsequent trailers in such an assembled combination are not literally semi-trailers because the combination of a dolly and semi-trailer results in substantially the full load of the trailer being independently supported by the dolly and trailer wheels. For the purpose of this invention, such rigs are also considered to be semi-trailers and the dolly may or may not be equipped with brakes. In addition, other combinations are common, such as are found in articulated and non-articulated busses and other types of articulated combination trucks. In addition, the invention is not limited to the type of air brake system found on such long haul trucks, and, in fact, may be used to monitor a wide variety of braking systems.
Malfunctioning brakes are the leading mechanical cause of commercial vehicle accidents and constitute the most common safety violation. Air brakes are used on most tractors and trailers with gross vehicle weight ratings of over 19,000 lb. (8600 Kg), most single trucks over 31,000 lb. (14,000 Kg), most transit and inter-city buses, and about half of all school buses. Commercial vehicle safety and accident analysis reveals numerous reasons why air brakes are such a problem; these reasons fall primarily into two categories related to the design characteristics of air brake systems. First, air brake systems are more sensitive to adjustment condition than hydraulic brakes. Second, air brakes provide less tactile warning of brake degradation to the driver than hydraulic brakes. Because accidents involving heavy trucks and buses have the potential to be severe, a means for detecting such problems is needed. Surprisingly, no commonly available brake warning systems are available that can warn drivers about a loss of brake effectiveness due to mechanical causes other than low supply pressure.
Data collected by the National Highway Traffic Safety Administration (NHTSA) from 1988 to 1990 show that whereas only 1 percent of registered vehicles are commercial vehicles, they represent 5 percent of total vehicle miles traveled. Furthermore, accidents involving commercial vehicles are estimated to account for approximately 10 percent of fatalities. Data from the Fatal Accident Reporting System, collected by NHTSA, from 1982 to 1990 show that of the 50,000 fatal accidents involving heavy trucks during that period, only 8400 (17 percent) of the fatalities were heavy truck occupants. The overwhelming majority (69 percent) of fatal injuries were caused to automobile or light truck occupants. Further studies have estimated that 40 percent of all trucks will be involved in a brake related crash during the lifetime of the truck and that in 33 percent of all truck accidents, a brake system problem is a contributing factor. These statistics underscore the importance to the general public of improved commercial vehicle safety.
The only existing means of determining brake condition is to measure brake chamber adjustment (stroke) on a stationary vehicle, which requires a wheel by wheel inspection by trained personnel. Chamber manufacturers provide specifications for acceptable stroke levels for various chamber sizes. Commercial vehicle enforcement officials nationwide use the Commercial Vehicle Safety Alliance North American Uniform Inspection and Out-of-Service Criteria for determining when to declare vehicles out-of-service (OOS) and hence inoperable. The Commercial Vehicle Alliance guidelines state that a vehicle must be put OOS if at least 20 percent of the vehicle's brakes are defective. One defective brake is defined as either one brake one-quarter inch or more beyond the readjustment point or two brakes less than one-quarter inch beyond the readjustment point. The vehicle must also be declared OOS if a steering axle brake is one-quarter inch or more beyond the adjustment limit, or if brake adjustment on two sides of a steering axle differ by one-half inch or more (because of concerns about steering wheel pull).
Commercial vehicle air brakes pose a significant safety concern on today's roadways for a number of technical reasons. Ninety percent of heavy truck and bus air brakes currently consist of drum type S-cam foundation brakes, with diaphragm chambers and manual or automatic slack adjusters. The diaphragm-type brake is very sensitive to adjustment condition; chamber pressure versus force characteristics are nonlinear, and there is a sudden drop-off in force when the pushrod stroke exceeds the recommended level. Chamber pushrod stroke increases as the brake shoes wear, or as the drums expand at higher temperatures. But when the recommended adjustment level is exceeded, the diaphragm diminishes in effective area as stroke increases, which, along with other kinematic and design-related factors, causes the braking force for a given pressure level to diminish sharply. When pushrod stroke becomes so great that the pushrod bottoms out in the chamber, brake force drops to zero.
In hydraulic brake systems, application of the brake pedal acts to pressurize a fluid so that the motion of the pedal displaces a fixed volume, and pedal height is proportional to brake adjustment. In contrast, application of an air brake pedal (treadle valve) simply opens a metering valve to divert compressed air from the storage tank(s) to the brake chambers. Hence, only a slight increase in brake pedal travel achieves greater delivered pressure.
The sensitivity of air brake systems to adjustment is compounded by a lack of feedback to the driver. Because air brake pedal height does not change appreciably with the amount of air used, the driver is insulated from direct energy input to the brakes, and as braking efficiency diminishes (through loss of adjustment, thermal loads, or other factors), very little tactile sensation is transmitted through the treadle valve. In other words, the brake pedal does not necessarily feel "spongy" or low, as in a typical automobile. The only real feedback a driver receives is the sensation of deceleration for a perceived pedal application position. The relatively large mass and low deceleration rates of commercial vehicles exacerbate the difficulty in perceiving brake degradation.
Adjustment sensitivity is further compounded by an increase in the time necessary for all the brakes to reach full operating pressure. As pushrod stroke increases, not only does the force level drop, but the brakes take longer to reach the desired application pressure (air transmission lag time). For properly adjusted brakes, it can take over half a second for adequate air pressure to reach the farthest axle of a triple trailer combination; this can add significantly to stopping distance. Tests have shown that application times can increase by about 80 percent when strokes go from the fully adjusted condition to the legal limit.
Because of the compounding effects of brake fade and drum expansion, hot brakes experience a significant reduction in braking performance. If only some of the brakes are properly adjusted, then those in adjustment will take a disproportionate share of the load, and may fade prematurely, shifting the load to the other (poorly adjusted) brakes. One study showed that for a fully adjusted brake operating at 600.degree. F. (355.degree. C.), the available brake torque is 85 percent of maximum, and it drops to only 50 percent of maximum when the stroke reaches the upper adjustment limit.
An additional factor complicates the understanding of brake performance as measured by pushrod stroke on a stationary vehicle. Pushrod stroke (at a given pressure) has been found to increase beyond the statically determined value when the vehicle is in motion. This phenomenon, called dynamic stroke increase, is believed to be caused by self-energization of the brake mechanism and elastic deformation of the foundation brake components. The dynamic stroke increase has been reported to be approximately 0.1 in. (0.25 cm) at 85 psi (60 N/cm.sup.2).
It was noted that there are several characteristics of braking a vehicle with an air brake system. One is that of the nature of deceleration. As with any friction brakes, deceleration is proportional to braking force applied divided by the mass (weight) of the vehicle. It is anticipated that vehicle weight will be provided automatically for vehicles, but could be input manually. From Newton's Second Law, the deceleration should be: EQU decel=1/m(F.sub.b +F.sub.d),
where:
F.sub.b =braking force PA1 F.sub.d =drag force PA1 M=vehicle mass PA1 deceleration; PA1 application pressure; PA1 response pressures; PA1 vehicle speed; PA1 weight; PA1 brake stroke; PA1 brake temperature; PA1 date; and PA1 time. PA1 brake lag is excessive, indicating excessive stroke; PA1 brake decay is excessive, indicating excessive stroke; PA1 measured values of deceleration for a single braking cycle are beyond a predetermined threshold of acceptable deviation for a single braking action from the model; or PA1 the composite average of deceleration and response delay for the current braking cycle and a predetermined number of previous cycles are beyond a weighted threshold of acceptable deviation from the model.
The braking force (F.sub.b) for each brake reduces to: EQU (F.sub.b)=c.sub.0 +c.sub.1 (temp)+c.sub.2 (stroke)+c.sub.3 (speed)+c.sub.4 (pressure))